Abstract: The LIGO/Virgo Collaboration (LVC) detected on January 4th, 2017 a
significant gravitational wave (GW) event (now named GW170104). We report in
this Letter the main results obtained from the analysis of hard X-ray and
gamma-ray data of the AGILE mission that repeatedly observed the GW170104
localization region (LR). At the LVC detection time $T_0$ AGILE observed about
36% of the LR. The gamma-ray imaging detector did not reveal any significant
emission in the energy range 50 MeV - 30 GeV. Furthermore, no significant
gamma-ray transients were detected in the LR that was repeatedly exposed over
timescales of minutes, hours and days. We also searched for transient emission
using data near $T_0$ of the omni-directional detector MCAL operating in the
energy band 0.4--100 MeV. A refined analysis of MCAL data shows the existence
of a weak event (that we call "E2") with a signal-to-noise ratio of
$4.4\,\sigma$ lasting about 32 ms and occurring $0.46\,\pm\,0.05 \,\rm s$
before $T_0$. A study of the MCAL background and of the false-alarm rate of E2
leads to determining a post-trial significance of $3.4\,\sigma$ for a temporal
coincidence with GW170104. We note that E2 has characteristics similar to those
detected from the weak precursor of the short GRB 090510. The candidate event
E2 is worth of consideration for simultaneous detection by other satellites. If
associated with GW170104, it shows emission in the MeV band of a short burst
preceding the final coalescence by 0.46 sec and involving $\sim 10^{-7}$ of the
total rest mass energy of the system.